Passively operated pigs are used for inspecting and cleaning pipelines, for example gas or oil pipelines. For the propulsion, which is carried out by the medium, the pigs are provided with collars which, in the direction of their longitudinal center axis, which also corresponds to the longitudinal center axis of the pipeline, cover the free pipeline cross section as well as possible. The medium presses against the collars and therefore drives the pig forward. Collars which are of substantially flat design in the direction of their planar extent are known, Said disk-shaped collars are also called “disks”. Collars which are bent or buckled toward the edges and rearward with regard to a preferred and forward direction of movement are referred to as “cups”. In this respect, the inner fastening region, at least with its front side pointing forward in the direction of movement, is in advance of the contact region located on the rearwardly bent, outer segment. The outermost edge region of said collars can even extend in a planar manner with respect to the pipe inner wall. The two types of collars are shown in DE 195 02 764 A1. Specifically in the case of these cups, but also in the case of the disks, there is the problem, in the event of a differential pressure increase because of, for example, pipe inside diameters which taper excessively and in which the pig is greatly slowed down or may become stuck, in particular in pipelines through which gas flows, that, in the event of a differential pressure increase, the friction of the collar against the pipe inner wall greatly increases. When the frictional force is overcome, this results in what is referred to as a forward shooting of the pig at a greatly increased speed which lies outside the tolerances predetermined for cleaning or measurement. Undesirable pressure fluctuations are caused in the pipeline by the pig becoming stuck and shooting forward.